Investigation of the genetic transmission of germline copies of murine retroviral genomes has led to the chromosomal mapping of more than 10 distinct loci. Studies with related laboratory mouse strains and Asian mice have been undertaken to describe the stability and wild mouse origin of specific loci and to identify additional genetic factors which alter in vivo virus expression and susceptibility to exogenous virus. Most recently, mendelian crosses are being analyzed to describe the genetic basis for the great differences in induction efficiency shown by the apparently allelic ecotropic loci of DBA and SEA mice. Studies on the transmission of the endogenous mouse mammary tumor virus (MMTV) in sexual crosses have resulted in the characterization of the complete and fragmented proviral genomes carried by several inbred strains. One full-length MMTV provirus was chromosomally localized near the MuLV locus, Bxv-1. Finally, studies with feral mouse populations have shown that many lack germline copies of ecotropic and xenotropic retroviruses. Cells of wild mice also differ from inbred mice in their susceptibility to exogenous infection, and genetic crosses are being used to analyze these differences. Preliminary studies have identified a new allele of Fv-1 in M. praetextus.